This invention relates to a servo writer for recording a servo signal on a servo band of a magnetic tape, a signal output control method, a signal output control program, and a manufacturing method of a magnetic tape using the signal output control method.
The rapid-paced advance of information storage technologies has been making storage media, such as magnetic tape, denser in recent years. High-density packing has been realized magnetically; for example, 400 gigabytes of data can be stored for computer backup on a reel of magnetic tape. For such high-density packing on a magnetic tape, hundreds of data tracks are arranged across the width of the tape. Accordingly, the widths of the data tracks have become very narrow, and the spacings between adjacent data tracks have become very narrow, too. In order to allow a read/write element of a magnetic head to trace the data tracks, servo control is exercised over the position (across the width of the magnetic tape) of the magnetic head, based on a servo signal that has been recorded beforehand on the magnetic tape and is read out by the magnetic head during reading/writing operations.
In a magnetic tape information recording/retrieving system, a point of change in magnetism in a servo signal is detected by a servo signal read element (magnetoresistive or MR element) detecting a change in electric resistance, and the point of change in magnetism is output as a read-out signal in a differential waveform (voltage value). Accordingly, the greater the change in the electric resistance of the MR element is, the greater a peak voltage value of the read-out signal in the servo signal becomes, which improves a signal-to-noise ratio of the read-out signal.
However, assuming that the oncoming stage of higher recording density of the magnetic tape to the order of ten terabytes arrives in the near future, according as the recording density becomes higher, the number of data tracks will increase, the widths of data tracks will become narrower, and the spacings between adjacent data tracks will become narrower, while the thickness of magnetic tape will become thinner. In consequence, the amount of detectable magnetism in the servo signal to be detected by the read element would decrease, and the peak voltage value of the read-out signal from the recorded servo signal would become smaller, so that the signal-to-noise ratio of the read-out signal would become lower. This trend would eventually deprive the magnetic tape information recording/retrieving system of its important feature of accurately reading a servo signal, thus disabling the system from controlling the position of the magnetic head at worst.
In order to improve the signal-to-noise ratio of a read-out signal to be acquired from a servo signal, a method as disclosed for example in JP 2004-318977 A (a family U.S. patent application also published under US 2005/0105967 A2) proposes magnetizing a magnetic tape along the length in one direction (one of lengthwise directions; e.g., a tape-transport direction) of the magnetic tape, to obliterate a servo band using an erasing head energized by a direct current (hereinafter referred to as direct-current erase or “DC erase”), and thereafter writing a servo signal on the servo band by forming a servo pattern magnetized in a direction opposite to the one direction in the servo band. When the servo signal recorded in accordance with this method is retrieved by a servo signal read element of the magnetic head, the quantity (rate) of change in magnetization of the magnetic field detected at the boundary between the ground (area magnetized in one direction, i.e., DC erased) and the figure (servo pattern formed by magnetization in a direction opposite to the one direction) is expected to be greater (higher), and thus the output of a read-out signal to be acquired from the servo signal is expected to be larger. As a result, the signal-to-noise ratio of the read-out signal to be acquired from the servo signal can be improved (see Paragraphs 0026-0027 and FIG. 1 of JP 2004-318977 A).
In order to stabilize an output value of a read-out signal to be acquired from a servo signal, a method as disclosed for example in JP 2005-85328 A (a family U.S. patent application also published under US 2005/0052779 A1) proposes controlling an erasing current supplied to a DC erasing head based upon the output value of the read-out signal acquired from the servo signal by a servo signal read element of the magnetic head. In this method, a recording current supplied to a DC erasing head or a servo signal write head is controlled such that an output value of the read-out signal to be acquired from the recorded servo signal is kept substantially constant, based upon the output value of the read-out signal acquired from the servo signal that has already been recorded. Accordingly, even when the magnetic tapes having a magnetic layer varying in thickness are supplied (i.e., how thin the magnetic layer is formed varies from tape to tape and even within the same tape), a servo signal can be recorded on a servo band thereof on an adequate level of quality, and the output value of the read-out signal to be acquired from the servo signal can be stabilized (see Paragraphs 0031-0034 and FIG. 2 of JP 2005-85321 A).
Various related techniques for improving a signal-to-noise ratio of a servo signal are known in the art, in which an area corresponding to a servo band on a magnetic tape is magnetized entirely in one direction along the length (i.e., the direction of magnetization or magnetic moment is one of lengthwise directions), and a servo signal is recorded by forming a servo pattern magnetized in a direction opposite to the one direction within the area, thereby forming the servo band. See, for example, US Patent Documents: U.S. Pat. No. 6,970,312 B2, US 2005/0099718 A1, US 2005/0168869 A1 and US 2005/0219734 A1.
However, the aforementioned techniques are all premised that a servo signal is detected with the envelope detection technique using an analog circuit, and variations which would appear in components, such as a capacitor, a diode, etc. making up the circuit for detecting a servo signal could cause problems in its reliability. Moreover, the circuit used for detecting a servo signal should be designed specifically for use with a particular type of device different among types of devices varying in servo patterns and/or in operating speeds, and thus it is often the case that a circuit designed to detect a servo signal in one device cannot be used by connecting with another device. Little flexibility in connecting with a different type of the device would be a disadvantage to be overcome.
In the technique as disclosed in JP 2005-85321 A, dropouts or read-out errors of a servo signal, which could occur for some reasons, would disadvantageously affect its recording current stabilizing feature, thus making it difficult to output a recording current controlled with high precision based upon variations in thickness of the magnetic layer.
Against this backdrop, the present invention has been made in an attempt to eliminate the above disadvantages. Therefore, apparatuses and methods consistent with the present invention may relate to a servo writer, a signal output control method, a signal output control program and a manufacturing method of a magnetic tape using the signal output control method, in which variations in components making up the circuit for detecting a servo signal can be reduced, and/or flexibility in connecting with a different type of the device can be improved, and/or a high-precision recording current can be output based upon variations in thickness of the magnetic layer.
Illustrative, non-limiting embodiments of the present invention overcome the above disadvantages and other disadvantages not described above. Also, the present invention is not required to overcome the disadvantages described above, and an illustrative, non-limiting embodiment of the present invention may not overcome any of the problems described above.